A bubble menu on link selection offers "edit URL", "open in new tab",
and "remove link"; Mod-k opens the same editor for the current
selection (creating a link if there isn't one yet), and the toolbar
button does the same. Invalid protocols (e.g. javascript:) show a
localized inline error instead of silently no-oping. Pasting a URL
over selected text links it instead of replacing the text.
Links always render with target="_blank" so read mode opens them in a
new tab by default; edit mode suppresses the resulting navigate-on-
click (Mod-click still follows it), since a plain click there should
place the cursor instead.
Closes#29
